1. Field of the Invention
The present invention relates to a display device, such as a liquid crystal display device, and to a control method therefor and particularly to an improvement in a display device equipped with a light sensor for monitoring outside illuminance.
2. Description of the Related Art
As display devices for information equipment including personal computers and word processors and for image equipment including television sets, camcorders and car navigation systems, liquid crystal display devices having features of lightweight, small thickness and low power consumption have widely been used. In the liquid crystal display devices, in order to realize a bright display screen, major devices adopt a configuration having a built-in backlight irradiating illuminating light from behind a display element.
One of the merits the liquid crystal display devices have is the property of low power consumption. Generally, electric power consumed by a backlight is greater than that consumed by a liquid crystal display panel. On the other hand, in the liquid crystal display devices, it is ordinarily necessary to heighten the display luminance as much as possible in order to sufficiently acquire visual discernibility and correspondingly thereto, when the backlight is set brightly, the property of low power consumption will possibly be diminished.
In view of the above, studies have been made on adjusting the brightness of a backlight in accordance with the environmental illuminance (outside illuminance) to suppress the power consumption as much as possible. The luminance required for acquiring sufficient visual discernibility in the case of low environmental illuminance as at night differs from that in the case of high environmental illuminance as out of doors in the fine daylight. It is conceivable that the illuminance of the backlight is adjusted to requisite minimum in accordance with the above difference, thereby enabling suppression of power consumption.
In order to realize the above, it has been proposed that in a liquid crystal display device, a light sensor (photosensor) is disposed outside the display region of a substrate and the output thereof is utilized to adjust the brightness of a backlight (refer, for example, to Japanese Patent No. 3015400, JP-A HEI 4-174819 and JP-A HEI 9-146073).
Japanese Patent No. 3015400 discloses a liquid crystal display device comprising first and second substrates, a liquid crystal layer sandwiched between the first and second substrates, a liquid crystal display panel having first and second photo acceptance units formed at the periphery of at least one of the first and second substrates, a backlight installed adjacently to the liquid crystal display panel, wherein the first photo acceptance unit detects the surrounding outside light and the second photo acceptance unit detects the light from the backlight, and means for adjusting the amount of light output from the backlight based on the results of detection by the first and second photo acceptance units.
JP-A HEI4-174819 discloses a liquid crystal display device comprising a liquid crystal display panel, a backlight for irradiating light from the back surface of the liquid crystal display panel, a controller for adjusting the amount of light from the backlight and a photodetector disposed abeam of the liquid crystal display panel, wherein the photodetector detects the surrounding brightness on the front surface of the liquid crystal display panel and the controller is adjusted to adjust the amount of light from the backlight.
JP-A HEI9-146073 discloses a backlight light-adjusting circuit for supplying luminance adjustment voltage to a backlight drive circuit of a liquid crystal display device having a backlight, comprising plural light sensors that detect the surrounding brightness on the front surface of a liquid crystal display panel and output outside illuminance signals, average value calculating means that calculates the average value of all or part of the outside illuminance signals output from these light sensors and luminance adjusting means that adjusts the luminance of the backlight drive circuit based on the average value of the outside illuminance calculated by the average value calculating means and the amount of the adjusted light set manually.
According to each of the related art references, it is made possible to adjust the display luminance in accordance with the outside illuminance and suppress the power consumption of mobile telephones or Personal Digital Assistants (PDAs). In each of the related art references, however, a configuration having a discrete light-adjusting sensor is installed independently of the liquid crystal display panel. This is contrary to the demands of the miniaturization and small thickness of liquid crystal display devices. Particularly, in the mobile telephones and PDAs, these demands are made severe. Thus, the prevention of the miniaturization and small thickness poses a serious problem.
It is conceivable that in order to solve the above problem a light sensor is integrally formed with one of the substrates (array substrate, for example) of a liquid crystal display panel using a low-temperature polysilicon technique, for example. However, the light sensor having been integrally formed with the array substrate using the low-temperature polysilicon technique has a problem that its characteristics vary depending on the surrounding temperature. Electric current flowing the light sensor includes a thermocurrent increasing or decreasing depending on the temperature besides a photocurrent proportional to incident light. For this reason, even when an output corresponding to 500 lx is obtained from the light sensor, it is difficult to adequately distinguish whether the output obtained is in fact 500 lx or the result that a great amount of photocurrents have flowed due to high temperatures in spite of 300 lx. Thus, a problem is posed in that light-adjusting performance fluctuates depending on the surrounding temperature.
The related art references do not sufficiently take measures to meet the situation with respect to variation or instability of an output from the light sensor and have a problem of difficulty in accurate luminance adjustment.
Since an illuminator, such as a fluorescent, emits light at a prescribed frequency, it induces so-called flicker, and the illuminance thereof varies with time. Therefore, a light sensor possibly detects light brighter or darker than actually detected. The error in such detection poses a serious problem in adjusting the luminance and possibly poses a problem of the display luminance failing to comply with the actual surrounding brightness.
The present invention has been proposed in view of the conventional state of affairs. One object thereof is to provide a display device capable of coping with the demands of the miniaturization and small thickness thereof, accurately measuring the illuminance with no temperature-dependency and appropriately controlling the luminance (the amount of light of a backlight, for example). Another object of the present invention is to provide a display device capable of accurately grasping the outside illuminance even when the illuminance varies with time, for example.